A considerable fraction of emissions from an internal combustion engine, perhaps as much as 80%, is generated during approximately the first 60 seconds of engine operation after the engine is started. There are two primary reasons for this. The first is that the catalyst is not yet functional and has low conversion efficiency, since it has not achieved its minimum threshold operating temperature of 300.degree. C. to 350.degree. C. The second reason is that the exhaust gas oxygen sensor used for feedback control of Air/Fuel (A/F) ratio is not yet functional, resulting in the need to use open loop or scheduled control rather than feedback control. Exhaust gas oxygen sensors may require as long as 60 seconds after power-on to become operational. Scheduled A/F ratio control is inherently less accurate than closed loop control because of variability and possible degradation in the functional characteristics of production components, leading to variability in system gains and dynamics.
During cold start (and even some hot start engine operation), the EGO sensor is not available or is not yet functional, resulting in the use of open loop or scheduled A/F control. Since accuracy under this condition is typically poor, it is desirable to improve the accuracy of open loop control to (1) reduce excessive emissions from excessively rich A/F operation (A/F&lt;&lt;14.65), and (2) reduce the occurrence of engine misfires and unstable operation from excessively lean A/F operation (A/F&gt;&gt;14.65).
Several different algorithms have been developed that estimate the A/F ratio for use in open loop or scheduled A/F control. These methods, however, require the use of an in-cylinder pressure sensor and high resolution crank angle calculations.